The present invention relates to cranes having hydraulically operated derricking booms.
In such cranes one or more hydraulic rams are normally positioned underneath the boom and are positively connected between the platform or base from which the crane boom pivots and the crane itself. The crane boom has therefore two points of attachment to the platform or base, one point being the ram attachment point and the other being the boom pivot point. During the manufacture of such cranes it is essential if the boom is to work at low angles as is the case in a mobile crane that the machining of the boom pivot holes relative to the derrick ram pivot holes or vice versa is carried out with a very tight tolerance. This is because at low elevations the angle between the derrick rams and boom is very small and very small differences in hole centres can cause large changes in the limiting angles of the boom. Maintaining the tight tolerance required is a costly and time consuming process.
A further problem is that stresses produced in the boom in the area about the ram attachment point are high and fluctuate from tension to compression due to high locally applied loads and moments. Whilst allowance for these stresses presents no particular problem, nevertheless when duty cycle work fatigue are design parameters, the penalties are significant. For example where the stress is an item changes from tension to compression the permissable working load may be only about 60% of that where the stress remains in tension or compression for any given number of working cycles. Similar disadvantages can also occur when the stress changes too rapidly even when only in tension or only in compression.